Magnetic resonance microscopy (MRM) is a powerful tool for probing structure and molecular dynamics on a microscopic scale. While the resolution of MRM is poor in relation to other modalities, such as electron microscopy, it has the advantage of being nondestructive and the ability to examine the dynamics of a system, and not only its inherent structure. MRM is fundamentally magnetic resonance imaging (MRI) on a smaller scale with the system redesigned to provide high resolution images. The major requirement to perform MRM over MRI is the need for a large increase in the signal-to-noise ratio (SNR). For example, in order to translate from a typical MRI isotropic resolution of 1 mm.sup.3 to a MRM scale resolution of 5 .mu.m.sup.3 requires an improvement in SNR per voxel of 8.times.10.sup.6. The SNR is not the only contributing factor to MRM resolution, however, with properties such as molecular diffusion, relaxation behavior, and sample-induced susceptibility boundary distortions affecting ultimate limits.
The method used to obtain SNR improvements is normally to decrease the size of the sample and associated hardware, namely radio frequency (rf) probes and gradient sets, and to increase the static field strength of operation. When small samples of rf coils are used, the resonant impedance of the coil usually dominates over sample impedance and the SNR is approximately proportional to B.sub.0.sup.7/4 where B.sub.0 is the static flux density in Tesla. Herein lies the requirement of MRM to operate in strong magnetic fields, typically 7 T (300 MHz proton precessional frequency) or higher is used for modern MRM.
Operating at high field strength is also of great advantage to nuclear magnetic resonance (NMR) spectroscopists due again to the increase in SNR but also the increased chemical shift dispersion. The use of high field systems for molecular structural determination considerably predates MRM. All known MRM systems use magnets designed for molecular structure determinations, and these magnets are typically very long relative to their bore size, offering little access to the sample under study. This limited access is a distinct disadvantage in many MRM applications. NMR magnets for high field applications typically consist of a set of coaxial solenoidal coils connected with two or more external "correction" coils in the fashion described some time ago by Garrett (M. W. Garrett, J. Appl. Phys. 22, 1091 (1951); M. W. Garrett, J. Appl. Phys. 38, 2563 (1967)). These layouts have formed the basis of even the most modern coil structures (T. Kamikado et al., IEEE Trans. Magn. 30, 2214 (1994); S.-T. Wang et al., IEEE Trans. Magn. 30, 2340 (1994)). The magnet designs disclosed in these publications are typically very long relative to their bore size, (usually having a total coil length to diameter of sensitive volume ratio of 20 or so) offering little access to the sample under study. This limited access can be a distinct disadvantage in many applications. A representative layout is shown in the cross section of FIG. 1, where each of the sections contains many thousands of turns of superconducting wire. The requirements of this design are that it is "homogeneous" over a diameter-sensitive-volume (dsv), where the conductors are operating with suitable factors of safety in terms of their critical current carrying capacity and the critical field within which they reside. For simplicity, only the coil structure is detailed in this specification and not the ancillary cryogenic structures.
It is the object of this invention to provide magnet designs for high field, high resolution spectroscopy and imaging that have a reduced length over conventional designs and still produce suitably homogeneous fields over a central volume while simultaneously allowing only a small amount of field leakage external to the magnet.